1. Field of the Invention
The present invention relates to a ladder filter including ladder-connected serial resonators and parallel resonators.
2. Description of the Related Art
A two-stage ladder filter used for communication equipment is shown in FIG. 1. In this ladder filter, two serial resonators 3 and 4 are connected in series between an input terminal 1 and an output terminal 2. A parallel resonator 5 is connected between the midpoint of the serial resonators 3 and 4 and a ground, and a parallel resonator 6 is connected between the output terminal 2 and the ground.
FIG. 2A is a vertical sectional view showing the specific construction of a conventional ladder filter, and FIG. 2B is a horizontal sectional view thereof. This ladder filter 11 includes an input terminal plate 12, a ground terminal plate 16, an output terminal plate 18, an internal connection terminal plate 14 which is bent into a U-shape, piezoelectric resonators 13 and 19 for defining serial resonators 3 and 4 utilizing extensional vibration (hereinafter referred to as xe2x80x9cextensional resonatorxe2x80x9d), and piezoelectric resonators 15 and 17 for defining parallel resonators 5 and 6 utilizing extensional vibration (extensional resonators). The input terminal plate 12, the ground terminal plate 16, and the output terminal plate 18 have lead feet 12a, 16a, and 18a, respectively. The extensional resonators 13, 15, 17, and 19 each perform extensional vibrations wherein the expansion toward the outer periphery direction and the contraction toward the center direction are repeated by the application of an electrical signal. Nodes are located at the centers of the main surfaces of each of these extensional resonators 13, 15, 17, and 19.
As shown in FIGS. 2A and 2B, this ladder filter is defined by stacking the above-described components in the order of the input terminal plate 12, the extensional resonator 13, one electrode 14a of the internal connection terminal plate 14, the extensional resonator 15, the ground terminal plate 16, the extensional resonator 17, the output terminal plate 18, the extensional resonator 19, the other electrode 14b of the internal connection terminal plate 14. Herein, protrusions provided on the input terminal plate 12, the ground terminal plate 16, the output terminal plate 18, and the electrode plates 14a and 14b of the internal connection terminal plate 14 are each abutted against the central portions which are the nodes of the extensional resonators 13, 15, 17, and 19. The lead feet 12a, 16a, and 18a of the respective input terminal plate 12, the ground terminal plate 16, the output terminal plate 18 are each inserted into holes of a bottom lid 21. The holes are filled with a resin 22, and sealed by providing a cover 20 thereon.
Such a ladder filter, however, not only has a complicated structure and is difficult to assemble, but also must be redesigned each time the number of stages thereof is increased. This redesign process is both time consuming and costly. For example, since the two-stage ladder filter and the three-stage ladder filter have very different terminal structures (particularly, in the structure of the internal connection terminal), it is impossible to design a three-stage or four-stage ladder filter on the basis of the structure of a two-stage ladder filter, and there is a need for redesigning whenever the number of stages is changed.
FIG. 3A shows the construction of an extensional resonator used as a serial resonator or a parallel resonator in the ladder filter as described above. FIG. 3B shows the directions of the polarization axis and the electric-field axis thereof. This extensional resonator 7 is provided with surface electrodes 9 on the main surfaces of a single-layered piezoelectric layer 8 having a square shape, and the entire piezoelectric layer 8 is polarized in a direction that is perpendicular to both main surfaces. Since the direction of an electric field applied across the surface electrodes 9 (the electric-field axis) is also perpendicular to both main surfaces, the electric-field axis is parallel with the polarization axis. In such an extensional resonator 7, once a signal is applied between the surface electrodes 9, the piezoelectric layer 8 expands and contracts with respect to the outer periphery direction, in the planes parallel with both main surfaces.
In the extensional resonator 7, the product of the length Ls of one side thereof and resonance frequency fr is substantially constant as expressed by:
Lsxc3x97fr=Asxe2x80x83xe2x80x83(1)
where, As is a constant (frequency constant), and approximately equal to 2100 mmkHz. For example, when trying to obtain a resonator having a resonance frequency fr=450 kHz, the length of one side thereof will be Ls=4.67 mm.
However, since there is an increasing need to miniaturize electronic components, it is difficult for such an extensional resonator to meet the needs for reductions in the size and weight, and further for cost reduction. That is, the dimensions as described above cannot be substantially reduced and maintain the desired resonance frequency.
FIG. 4 shows attenuation characteristics of the ladder filter having a two-stage configuration. As characteristics of such a ladder filter, the guaranteed attenuation value Att. shown in FIG. 4 must be as large as possible. Denoting the inter-terminal capacities of the serial resonators 3 and 4 as C1 and C1, respectively, and the inter-terminal capacities of the parallel resonators 5 and 6 as C2 and C2, respectively, the guaranteed attenuation value Att. of the ladder filter of a two-stage configuration is expressed by:
Att.=2xc3x9720 log (C2/C1)xe2x80x83xe2x80x83(2)
To increase the guaranteed attenuation value, therefore, it is necessary to increase the inter-terminal capacities C2 and C2 of the respective parallel resonators 5 and 6, and to decrease the inter-terminal capacities C1 and C1 of the respective serial resonators 3 and 4. However, when extensional resonators as described above are used as the parallel resonators 5 and 6, it has been difficult to increase the inter-terminal capacity C2, for the reasons described hereinafter.
Denoting the length of one side of the extensional resonator 7 shown in FIG. 3A as Ls, the dielectric constant of the piezoelectric layer 8 as xcex5, and the thickness thereof as t, the inter-terminal capacity Cs thereof is expressed by the following equation:
Cs=(xcex5xc3x97xcex50xc3x97Ls2)/txe2x80x83xe2x80x83(3)
where, xcex50 is a permittivity in a vacuum, and xcex50=8.854xc3x9710xe2x88x9212.
Since the length of one side of the extensional resonator 7 is determined if the resonance frequency fr of the extensional resonator 7 is determined (see the equation (1)), the inter-terminal capacity is determined only by the thickness t and the dielectric constant xcex5 of the piezoelectric layer 8.
To increase the inter-terminal capacity Cs of the extensional resonator 7, it is necessary to increase the dielectric constant xcex5 of the piezoelectric layer 8, or reduce the thickness t thereof. However, the dielectric constant xcex5 of the piezoelectric layer 8 is a constant inherent in the material of the piezoelectric layer 8, and cannot be optionally selected. If the piezoelectric material is changed to increase the dielectric constant xcex5, other characteristics are affected. On the other hand, if the thickness t of the piezoelectric layer 8 is reduced, the strength thereof will decrease, and the extensional resonator 7 becomes more susceptible to failure, so that the range of selection of the thickness t is substantially limited.
Therefore, although a resonator having a large inter-terminal capacity has been required as a parallel resonator for a ladder filter, it has been difficult to obtain a resonator having a large inter-terminal capacity. Furthermore, even if a piezoelectric resonator having a small constant corresponding to the above-described constant Cs is developed, and the miniaturization of the piezoelectric resonator is effected, the inter-terminal capacity will substantially decrease, and hence the guaranteed attenuation of a ladder filter will deteriorate when the piezoelectric resonator is used as a parallel resonator.
For a resonator operating in 300 kHz to 800 kHz band, extensional vibrations of a ceramic piezoelectric body have been utilized. FIG. 13A is a perspective view illustrating the construction of a piezoelectric resonator 101 utilizing extensional vibration (extensional resonator), and FIG. 13B is a side view illustrating the directions of the polarization axis and the electric-field axis thereof. This extensional resonator 101 is provided with surface electrodes 103 on both main surfaces of a single-layered piezoelectric layer 102 having a square shape, and the entire piezoelectric layer 102 is polarized in the direction perpendicular to both main surfaces. Since the direction of an electric field applied across the surface electrodes 103 (electric-field axis) is also perpendicular to both main surfaces, the electric-field axis is parallel with the polarization axis. In such an extensional resonator 101, when a signal is applied between the surface electrodes 103, the piezoelectric layer 102 expands and contracts with respect to the outer periphery direction, in the planes parallel with both main surfaces.
In the extensional resonator 101, the product of the length Ls of one side thereof and resonance frequency fr is substantially constant as expressed by:
Lsxc3x97fr=As
where, As is a constant (frequency constant), and substantially equal to 2100 mmkHz. For example, when trying to obtain the resonator of which the resonance frequency is fr=450 kHz, the length of one side will be Ls=4.67 mm.
However, since there is an increasing need to miniaturize electronic components, it is difficult for such an extensional resonator to meet the needs for reductions in the size and weight, and further for cost reduction. That is, the dimensions as described above cannot be substantially reduced and maintain the desired resonance frequency.
FIG. 14 shows a two-stage ladder filter 106 including serial resonators 107a and 107b, and parallel resonators 108a and 108b, and FIG. 15 shows attenuation characteristics thereof. As characteristics of this ladder filter 106, the guaranteed attenuation value Att. shown in FIG. 15 must be as large as possible. Designating the inter-terminal capacities of the serial resonators 107a and 107b as C1 and C1 respectively, and the inter-terminal capacities of the parallel resonators 108a and 108b as C2 and C2, respectively, the guaranteed attenuation values Att. of a ladder filter 106 of two-stage configuration is expressed by:
Att.=2xc3x9720 log (C2/C1)
In order to increase the guaranteed attenuation value, therefore, it is necessary to increase the inter-terminal capacities C2 and C2 of the respective parallel resonators 108a and 108b, and to decrease the inter-terminal capacities C1 and C1 of the respective serial resonators 107a and 107b. However, when extensional resonators 101 as described above are used as parallel resonators 108a and 108b, it has been difficult to increase the inter-terminal capacity C2, for the reason described hereinafter.
Denoting the length of one side of the extensional resonator 1 shown in FIG. 13A as Ls, the dielectric constant of the piezoelectric layer 102 as xcex5, and the thickness thereof as t, the inter-terminal capacity Cs is expressed by the following equation:
Cs=(xcex5xc3x97xcex50xc3x97Ls2)/t
where, xcex50 is a permittivity in a vacuum, and xcex50=8.854xc3x9710xe2x88x9212.
Since the length of one side of the extensional resonator 101 is determined if the resonance frequency fr of the extensional resonator 101 is determined (see the equation (1)), the inter-terminal capacity can be changed only by the thickness t and the dielectric constant xcex5 of the piezoelectric layer 102.
To increase the inter-terminal capacity Cs of the extensional resonator 101, it is necessary to increase the dielectric constant xcex5 of the piezoelectric layer 102, or reduce the thickness t thereof. However, the dielectric constant xcex5 of the piezoelectric layer 102 is a constant inherent in the material of the piezoelectric layer 102, and cannot be optionally selected. If the piezoelectric material is changed to increase the dielectric constant xcex5, other characteristics will be affected. On the other hand, if the thickness t of the piezoelectric layer 102 is reduced, the breaking thereof is substantially decreased, and the extensional resonator 101 becomes susceptible to failure, so that the range of selection of the thickness t is substantially limited.
Therefore, although a resonator having a large inter-terminal capacity is required as a parallel resonator of a ladder filter, it has been difficult to obtain a resonator having a large inter-terminal capacity. Furthermore, even if a piezoelectric resonator having a small constant corresponding to the above-described constant Cs is developed, and the miniaturization of the piezoelectric resonator is effected, the inter-terminal capacity substantially decreases, and hence the guaranteed attenuation of a ladder filter deteriorates when the piezoelectric resonator is used as a parallel resonator.
To overcome the above-described problems, preferred embodiments of the present invention provide a low profile and small-sized ladder filter. Another preferred embodiment of the present invention provides a ladder filter having a large guaranteed attenuation value and having superior characteristics.
In accordance with a first preferred embodiment of the present invention, a ladder filter is provided which includes a substrate, conductive members, plural sets of units each of which is defined by stacking a serial resonator utilizing bending vibration and a parallel resistor utilizing bending vibration with the conductive member sandwiched between one side surface electrode of the serial resonator and one side surface electrode of the parallel resonator, conductive patterns provided on the substrate, and a conductive lid for covering the units. In this ladder filter, the plural sets of units are two-dimensionally arranged on the substrate, the lid is arranged on the substrate so as to cover the units, the other side surface electrode of the serial resonator in each of the units at a second stage and at stages thereafter is conductive to the conductive member at the preceding stage by the conductive patterns and the lid, and the other side surface electrodes of the parallel resonators of all of the units are conductive to one another.
It is preferable that the above-described serial resonator and the above-described parallel resonator each have a different number of internal electrodes, with the resonator having the greater number of internal electrodes defining a parallel resonator, while the resonator having fewer internal electrodes defining a serial resonator.
Preferably, the other side surface electrodes of the parallel resonators of all of the units are conductive to one another via the lid of which substantially the entire surface is electrically conductive.
In the ladder filter in accordance with the first preferred embodiment of the present invention, since a plurality of units including the stacked serial and parallel resonators with the conductive member sandwiched therebetween, are two-dimensionally arranged on the substrate, the serial resonators and parallel resistors constituting each of the units are ladder-connected. The increase in the number of the stages of ladder filters is achieved by installing additional units on the substrate, and by making the other side surface electrode of the serial resonator in each of the units (this other side surface electrode is the surface electrode opposite to the surface electrode in contact with the conductive member, and constitutes the signal input side) conductive to the conductive member (signal output side) in the preceding-stage unit, as well as by making the other side surface electrode (ground side) of the parallel resonator in one of the units conductive to the other side surface electrode of the parallel resonator in another unit. Therefore, the wiring pattern is achieved by repeating a fixed pattern, which facilitates redesign when increasing the number of stages of a ladder filter. In particular, the use of the conductive members is shared even if the numbers of the stages of ladder filters are different.
Since the serial resonators and the parallel resonators in this ladder filter utilize area bending vibration, the dimensions thereof can be greatly decreased as compared to that of an extensional resonator. Also, since all of the units in the ladder filter are connected to one another by the wiring patterns provided on the substrate and the lid, the need for wiring spaces for interconnecting the units by bonding wire or leads is eliminated. This permits a very low profile and small-sized ladder filter to be produced.
Furthermore, in the ladder filter in accordance with the first preferred embodiment of the present invention, since, out of the resonators utilizing area bending vibration, a resonator including more internal electrodes is used as a parallel resonator, while the resonator including fewer internal electrodes is used as a serial resonator, the inter-terminal capacity of the parallel resonator is substantially greater than that of the serial resonator without substantially enlarging or substantially thinning the parallel resonator. This results in an increased guaranteed attenuation value and improved filter characteristics.
Moreover, in the ladder filter in accordance with the first preferred embodiment of the present invention, since the other side surface electrodes of the parallel resonators of all of the units are made conductive with respect to one another through the lid, there is no need to provide the substrate with conductive patterns for making the other side surface electrodes of the parallel resonators conductive to one another, and thereby the conductive patterns are greatly simplified. Even when reducing the size of the ladder filter, therefore, the line width of a pattern can be enlarged to reduce wiring resistance. In addition, since substantially the entire surface of the lid has an electrical conductivity, and is conductive to the other side surface electrodes of the parallel resonators, the other side surface electrodes having ground potentials, the lid has an electromagnetic shielding effect. This produces a ladder filter having a high noise immunity.
In accordance with a second preferred of the present invention, a ladder filter is provided which includes a substrate, a plurality of piezoelectric resonators utilizing area bending vibration, a lid for covering the piezoelectric resonators, conductive patterns provided on the substrate, and conductive members provided on the lid. In this ladder filter, the plurality of the piezoelectric resonators are two-dimensionally arranged on the substrate, the lid is mounted on the substrate to cover the piezoelectric resonators, and the plurality of the piezoelectric resonators are ladder-connected by the conductive patterns and the conductive members.
It is preferable that, by placing the nodes or the vicinities thereof of the piezoelectric resonators on the conductive protrusions disposed on the top surface of the substrate, the piezoelectric resonators is supported in a state isolated from the substrate, as well as the conductive patterns and the electrodes of the piezoelectric resonators be made conductive to one another via the conductive protrusions, and that, by abutting the conductive protrusions disposed on the bottom surface of the lid against the nodes or the vicinities thereof of the piezoelectric resonators, the lid being isolated from the piezoelectric resonators, as well as the conductive members and the electrodes of the piezoelectric resonators be made conductive to one another via the conductive protrusions.
It is preferable that the plurality of the above-described piezoelectric resonators utilizing area bending vibration be defined by resonators having a different number of internal electrodes and the resonator having the greater number of internal electrodes defines a parallel resonator, while the resonator having fewer internal electrodes is used as a serial resonator.
Preferably, the lid is formed by insert-molding metallic plates in a resin molded portion.
Preferably, the portions of each of the metallic plates, the portions which are exposed outside of the lid, are in contact with the conductive patterns on the substrate.
In the ladder filter in accordance with the second preferred embodiment of the present invention, since a plurality of piezoelectric resonators are arranged in two dimensions, and the piezoelectric resonators are ladder-connected by the conductive patterns on the substrate and the conductive members provided on the lid, the profile of the ladder filter is greatly reduced. Also, since the piezoelectric resonators utilize bending vibration, the dimensions of each of the resonators is greatly reduced, and a ladder filter having greatly reduced dimensions is achieved. This produces a much less expensive ladder filter.
Furthermore, in the ladder filter in accordance with the second preferred embodiment of the present invention, since the piezoelectric resonators are isolated from the substrate and the lid by sandwiching the nodes or the vicinities thereof of the piezoelectric resonators by the conductive protrusions provided on the substrate and the lid, vibrational spaces are provided over both surfaces of the piezoelectric resonators, and the vibration of the piezoelectric resonators is not substantially damped.
Moreover, in the ladder filter in accordance with the second preferred embodiment of the present invention, since among the piezoelectric resonators utilizing area bending vibration, the piezoelectric resonator including more internal electrodes is used as a parallel resonator while the piezoelectric resonator including less internal electrodes is used as a serial resonator, the inter-terminal capacity of the parallel resonator is larger than that of the serial resonator without the need to substantially enlarge or substantially thin the parallel resonator. This results in a greatly increased guaranteed attenuation value and greatly improved filter characteristics.
In addition, in the ladder filter in accordance with the second preferred embodiment of the present invention, since the lid is formed by insert-molding metallic plates in the resin molded portion thereof, the insulating portions and the conductive portions thereof are each produced easily and at a low cost.
Further, in the ladder filter in accordance with the second preferred embodiment of the present invention, since the portions of each of the metallic plates which are exposed to the outside of the lid are in contact with the conductive patterns of the substrate, it can be easily checked, from outward appearance, whether the metallic plates (conductive portions) of the lid are connected to the conductive patterns on the substrate.
The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.